Non-Blockchain interoperability. Off-Chain & Middleware systems. 

A way to facilitate interoperability between system is with off-chain or middleware systems. 

Atomic Swaps. 

Komodo

-Decentralized way to exchange two assets without going through a centralized exchange. 

-Real life barter system. 

(work on exchange of non-like protocols)

Oracles

Chainlink Oracles

-Can be used as a general purpose bridge between blockchains. 

-Transactional activity of one blockchain can be used as an input trigger to an on-chain smart contract

-Oracles enable a wide range of cross-communication not possible in other models.

State Channels

-Method for swaping assets on the same blockchain is by creating an off-chain state channel to transfer assets

-Allows different parties to trade and record ownership

-On-chain transactions are reserved for final settlement. 

-Scalable and beneficial for reducing on-chain transaction cost. 

-State channels can be used to exchange messages about state changes before settling on-chain

Blockchain Operating System

Quant

-Patented protocol that runs on top of blockchains. 

-Enables cross chain messaging and atomic swaps. 

-Sandbox testing environments. 

-Smartcontracts can be written in one language and deployed across many blockchains at the same time. (MAPPs)

-Contracts can be quickly adjusted to work on different platforms without needing rewrite. 

For more information read Interoperability and Connectivity: Unlocking Smart Contracts 3.0